1. Field of the Invention
The present invention relates to an encapsulant composition and, more particularly, to an encapsulant composition that can be used for a solid state light emitting device.
2. Description of Related Art
In the recent years, light emitting diodes (LEDs) have been widely used in many electronic devices such as LCD back-light sources, large displaying devices, and light illuminating equipment. Hence, demands for LEDs with high brightness have increased. Owing to the environmental requirement of various LED application, an encapsulant composition suitable for being used in light emitting devices should have certain characteristics such as being homogeneous, having high transparency, and being capable of long term storage capacity in order to resist to the high temperature when a large electrical current is applied. Conventional transparent encapsulant materials used in LEDs include epoxy or silicone resins, in which the cost of silicone resin is high though it has superior optical characteristics. In contrast, epoxy resin has been widely used due to its low cost. However, epoxy resin has a serious problem that yellowing occurs when it is used as the encapsulant composition of the LEDs. The yellowing of the epoxy resin, caused by the high temperature of the high power operation may influence the transmittance to the short-wavelength light and causing color shift. Therefore, epoxy resin is limited in its application for being used in a high current and high temperature operating environment.
The anti-yellowing of the encapsulant composition such as the epoxy resin is usually obtained by adding an antioxidant. The added antioxidant can remove the peroxide radical to stop the chain reaction, or reduce the degradation speed of the polymers by decomposing the hydroperoxide generated during the degradation. However, it is proved that the anti-yellowing obtained by adding of the antioxidant cannot be satisfactory for long operating time of the LEDs, not only suitable anti-oxidant is hard to find but also some anti-oxidants have high volatility that may result in a migration problem within the encapsulant material.
Besides, internal stress caused by high temperature during high power operation may contribute negative influence to the semiconductor element, cause a short circuit, and reduce the brightness of the applied LEDs. Therefore, for reducing the internal stress of the encapsulant composition, U.S. Pat. No. 5,145,889 has disclosed four methods, which comprise: (1) decreasing the glass transition temperature (Tg) of the encapsulant composition; (2) decreasing the linear expansion coefficient of the encapsulant composition; (3) decreasing the Young's modulus of elasticity (E); and (4) decreasing the shrinkage factor (ε). In general, decreasing of the glass transition temperature (Tg) will lower the mechanical strength of the encapsulant at high temperature; decreasing of the Young's modulus of elasticity will dramatically decrease the adhesion of the encapsulant to chip, board, or lead frame; besides if the amount of filler increase significantly, the viscosity of the encapsulant will increase too much and is unfavorable to the workability.
In U.S. Pat. No. 6,800,373, an encapsulant composition is disclosed, which comprises silicone resin, low molecular weight of alicyclic compounds, aromatic compounds, hetero-cyclic compounds which having epoxy resin functional group, and siloxane surfactant. However, the optical transparency is unsatisfactory, and the siloxane surfactant is neither involved in the reaction nor does it form a homogeneous mixture with common epoxy encapsulant composition.
Therefore, it is desirable to provide an improved encapsulant composition that can be used for a solid state light emitting device to achieve low internal stress and better anti-yellowing performance.